The invention relates to an automatic coupling comprising two parts which are supported inside one another, which are rotatable around a common longitudinal axis and which form an annular chamber filled with a highly viscous fluid, for the purpose of generating a locking effect between the two parts when the two parts rotate relative to one another. In referring herein to a "locking" effect between the two parts, it is to be appreciated that in practice the effect is one of an inhibition or braking of relative rotation between the parts rather than a complete prevention of relative rotation, and the term "locking" is to be interpreted accordingly.
Couplings of this type are known as viscous couplings and described in GB 1 357 106. These couplings are used in combination with differential drives or on their own in the drivelines of motor vehicles. In the former application they generate a differential-speed-dependent locking effect at the axle differentials or in central differentials. In the latter application they serve as so-called visco-transmissions which have the function of a differential-speed-dependent engageable coupling for the second driving axle which is normally the rear axle of motor vehicle with a permanent front wheel drive.
Furthermore, a coupling of this type is known from DE 37 25 103 C1 wherein the coupling plates of a viscous coupling operate in a highly viscous fluid and wherein a conveying worm also operating in the highly viscous fluid is intended to reduce the locking effect as a function of the fluid level in the region of the coupling plates in the case of a relative rotation.
Furthermore, DE 37 43 434 C2 describes a friction coupling operated by a pressure agent, which is combined with a viscous coupling, with the latter being included in the power flow between a housing and a hub when the friction coupling is loaded, whereas the parts rotate freely relative to one another when the friction coupling is not loaded.
Finally, P 43 43 307.3 proposes a Visco-Lok coupling wherein a highly viscous fluid in a chamber--as a result of shear processes in the highly viscous fluid--increases the pressure in such a way that there is displaced a piston which delimits the chamber and which loads a conventional multi-plate friction coupling in the sense of closing same.